1. Technical Field
The present invention relates to an improved process for end-capping anionic polymers, such as those of styrene, isoprene and butadiene formed by organolithium initiated polymerization including steps of reacting first with a steric hindrance agent such as 1,1-diphenylethylene, and then contacting with carbon dioxide in a solvent containing polar diluent such as tetrahydrofuran.
2. Background Information
Organolithium-initiated, anionic polymerization is a well-known synthetic method of preparing polymers, of such monomers as styrene, isoprene and butadiene, by which the major variables affecting polymer properties can be controlled. Under the preferred operating conditions generally known in the art, spontaneous termination or chain-transfer reactions of the forming polymer can be essentially avoided. This characteristic has led to standard use of the terms "living polymerization" or "living polymers" for such polymerization reactions and the corresponding polymers. Living polymerization allows the preparation of polymers of pre-determined molecular weight, narrow molecular weight distribution ("MWD"), and chain-end functionality. A vast array of synthetic procedures and novel polymers have resulted from the use of these well-characterized, functionalized polymers in grafting, copolymerization, and linking reactions Specifically, the carbonation of living polymeric anions using carbon dioxide is both known and in wide-use. However, as indicated in the prior art, substantial problems in achieving efficient conversion of living polymer to chain-end functionalized polymers without the undesirable formation of reaction by-products, both ketones and alcohols form joined or coupled polymeric chains, have arisen
In "Reaction of Polystyryllithium with Carbon Dioxide", Wyman, et al., Journal of Polymer Science; Part A, Vol. 2 pages 4545-4550 (1964), reported that polystyryllithium terminated with gaseous carbon dioxide y ielded the polystyrenecarboxylic acid but also di-polystyryl ketone and branched tri-polystyryl carbinol in a 60/28/12 % yield, respectively Mansson in "Reactive of Polystyryl Anions with Carbon Dioxide and Oxygen", Journal of Polymer Science, Polymer Chemistry Edition, Vol. 18, pages 1945-1956 (1980) reported yields of polystyrenecarboxylic acid lower than that reported by Wyman, et al, when the reaction of gaseous carbon dioxid e and polystyryllithium at about 10.degree. C. was conducted in a solvent of mixed methylcyclohexane and tetrahydrofuran (THF), as compared to the benzene solvent of Wyman. The authors cautioned that differing conditions required care in comparison but concluded generally that the ability of THF to dissociate dimeric monomeric species had no "dramatic influence on the yield of carboxylic acid."
Young, et in "Advances in Polymer Science" #56, pages 70-72, noted that use of Lewis bases such as tetrahydrofuran ("THF") served to promote disaggregation of polymeric organolithium species and thus in the presence of an excess of THF, in a 75/25 mixture by volume of benzene a nd T H F, car b o nation of poly (styryl) lithium, poly(isoprenyl)lithium, and poly(styrene-b-isoprenyl)-lithium, reportedly resulted in quantitative suppression of coupling side reactions It is suggested that the amount of THF used by earlier researchers was probably not sufficient to achieve disaggregation. Similar results are reported by Quirk, et al., in "Functionalization of Polymeric Organolithium Compound Carbonation" Makromolecular Chemistry, 183, 2071-2076 (1982). One hundred peroent yields were reported for 75/25 benzene/THF oarbonation solvents. Remarks as to the expressed need for oontaminate-free conditions were later discounted in Quirk and Yin, "Functionalization Reactions of Poly(styryl)-lithium with Carbon Dioxide", Polymer Preprints 29, 401-402 (1987). Carboxylation yields for the earlier report are he re characterized a s having been "essentially quantitative." Both reports teach polymerization at 30.degree. C. under high vacuum conditions with gaseous CO.sub.2 introduced after addition of THF, where utilized, into the polymerization reaction vessel The essentially quantitative yields were said to be obtained from freeze-dried solutions of poly(styryl)lithium.
It is further generally known in the art that certain compounds, including 1,1-diphenylethylene, can be used to modify the subsequent reaction of lithiated polymer chains. U.S. Pat. No. 3,890,408 teaches the use of CH.sub.2 =CR.sub.1 R.sub.2 compounds, including 1,1-diphenylethylene, to modify the terminal dienyl anion of living conjugated diene elastomer polymers for su bsequ ent block polymerization with 1-alkyl-ethylene carboxylic esters, such as methyl methacrylate. The diene elastomer polymers typically include polybutadiene, polyisoprene; copolymerization of butadiene or isoprene with such as styrene is suggested. It is suggested that addition of the 1,1-diphenylethylene serves to create a sterically hindered dienyl anion that no longer is susceptible to secondary reactions, such as reaction with the carbonyl group of the 1-alkyl-ethylene-carboxylic ester, or premature termination and crosslinking.
U.S. Pat. No. 4,068,050 teaches the preparation of macromolecular monomers (from living polymers reacted with "capping agents") for use in condensation type copolymerization reactions. The "capping agents" comprise lower alkylene oxides, 1,1-diphenylethylene, and conjugated dienes such as butadiene and isoprene. These "capping agents" are said to reduce the reactivity of the living polymers so as to allow reaction with a halogen-containing terminating agent of the halogen site rather than at the difunctional carboxylic acid-ester groups, e.g., with diethyl-2-bromo-2 methyl maleate. The terminated living polymer is the macromolecular monomer of the invention, it is comprised of such monomers as styrene, isoprene and butadiene, and is characterized by a Mw/Mn ratio which is not substantially above 1.1.
In view of the many uses for living polymers, particularly those comprised of styrene, isoprene and butadiene, having reactive end-groups capable of subsequent coupling, crosslinking, etc., the need for efficient, cost-effective means of preparation of such polymers is evident. It is thus an object of this invention to provide an improved method of functionalizing polymers produced by living polymerization whereby improved utilization of reactants is achieved while simultaneously minimizing the occurrence of undesirable side reactions and by-products.